I’ve found that the only way to do it is to copy and paste the function library node from another blueprint. However, it is quite awkward even doing that, because sometimes it will say “Conflicting nodes substituted during paste!” and not paste anything.
However, as long as I place the node somewhere inside of any function library with no nodes connected, then copy it, and then paste inside of the Object blueprint, it will work.
The one issue is that on compiling the Object blueprint, you will get a warning that using that node in an Object is “unsafe”, but as far as I can tell you can just ignore it.
I have no idea why it’s “unsafe” and why it’s so convoluted to do it at all, but in the absence of anyone in-the-know commenting with an actual reason, I am inclined to believe it’s just fine, because I’ve as yet encountered no side effects of this “unsafe” implementation.